1. Field of the Invention
The present invention relates to a cylinder block for an internal combustion engine, and more particularly, to a cylinder block structure capable of suppressing deformation of a cylinder bore and a gasket seal surface caused when fastening a cylinder head to the cylinder block.
2. Description of the Related Art
In conventional internal combustion engines as illustrated in FIGS. 10, 11 and 12, when a cylinder head 52 is fastened to a closed deck-type cylinder block 51 with a head bolt 53, a bolt boss 56 is pulled upwardly. This causes a moment E.sub.1, E.sub.2 about a rigid grommet 55 of a head gasket 54 in a plane connecting a bore center and a bolt center and, consequently causes deformations of the cylinder bore wall 57 and an upper deck 55. In this instance, intermediate cylinder bores cause a fourth-mode deformation and end cylinder bores cause a third-mode deformation as shown by the dashed lines in FIG. 12. In addition, the upper deck 58 inclines inwardly and downwardly as shown in FIG. 10. The deformation of the cylinder bores increases oil consumption and piston slap noise.
To suppress the cylinder bore deformation, various proposals have been made. For example, Japanese Utility Model Publication SHO 59-24846 proposes a cylinder block shown in FIG. 13, wherein a cylinder block outside wall 61 and a common wall portion 62 of a siamese bore wall structure are connected via a single bridge structure 63 on a side of an oil-ring of a piston to thereby suppress the fourth-mode deformation of the cylinder bore near the oil-ring. No deck is provided above the single bridge structure to thereby cut transmission of a bending moment through the upper deck.
However, there are problems with the conventional cylinder block. More particularly, although the cylinder bore deformation is suppressed, the upper end surface 64 of the cylinder block outside wall is inclined seriously by the fastening force of the head bolts 65 to cause a sealing problem. When the bolt boss 67 is pulled upwardly relative to the common wall portion located between adjacent cylinder bores, the cylinder block outside wall 61 falls inwardly as shown in FIG. 14. As a result, a gap g is generated between an upper end surface of the common wall portion and a lower surface of the cylinder head 68, through which gas will blow-by between adjacent cylinders. Further, the inclination of the upper end surface of the cylinder block outside wall may cause leakage of cooling water and may decrease gasket durability.